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Surface Engineering of Metals - Principles, Equipment and Technologies Part 3

Tham khảo tài liệu 'surface engineering of metals - principles, equipment and technologies part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | where a - emission constant dependent on material properties and on condition of surface emitting electrons differing in practice from the theoretical value. apract. 30 to 120 A cm2 K2 atheor. 4pemk2 h3 1.2 106 A cm2-K2 m - electron mass m 9.109-10-31 kg h - Planck constant h 6.2 10-34 J-s e - elementary charge e 1.602 10-19 C e - base of natural logarithms e 2.71828 A ey - work done by electrons exiting the emitting body eV j - exit potential V k - L. Boltzmann constant k 8 105 eV K kT - mean energy of electrons. The formula 2.1 is known as the Richardson or Richardson-Dushman equation. In order to attain optimum density of the thermoemission current which is approximately 0.1 to 1.5 -10-s A cm2 it is essential to generate a high temperature of the emitting body of the order of 2400 to 2700 C 7 . Materials used for emitting electrons can only be those with a high melting point i.e. pure metals borides oxides. Thermal emission materials utilized in electron beam equipment designated for surface enhancement are most frequently tungsten tungsten coated with thorium or with lanthanum hexaboride in the second case the essential current density can be attained at temperatures lowered to 1600 to 2000 K 1 4-6 . In practice the intensity of the thermoemission current depends on material and design of the element as well as on treatment conditions. Its value is usually within the range of 10-6 to 5 A 1 6 . 2.2.3 Utilization of plasma as a source of electrons Plasma1 is an electrically conductive thinned and ionized gas with a sufficiently high concentration of charged particles containing basically the same number of electrons and positive ions. It is a quasineutral mixture. Each substance may transcend to the state of plasma as the result of thermal ionization occurring at an appropriately high temperature. For this reason to this day plasma is sometimes called the fourth state of matter. With a rise in temperature there come about transitions solid liquid gas plasma. .